1. Field of the Invention
The present invention relates to a method for forming a single crystal semiconductor thin film which constitutes a so-called SOI (Silicon on Insulator) structure of an electronic circuit formed on a dielectric substrate.
2. Description of the Related Art
Various methods for forming the SOI structure of single crystal semiconductor thin film have been proposed. However, it has been difficult to form the single crystal thin film over a wide area on the substrate. For example, one of the methods is such that an energy beam such as a laser beam is irradiated to a polycrystalline silicon layer formed over an entire surface of a dielectric substrate so as to recrystallize the silicon layer. However, the crystal growth starts from the peripheral portion of the area where the beam is irradiated toward the centrer portion of the area so that a number of cores are formed. Therefore, crystal faces of the cores encounter each other, which impedes the growth of a large single crystal.
Also, in the melt and recrystallization process, the outer surface of the semiconductor thin film is being in contact with the air which has a low thermal conductivity. As a result, the heat radiation effect from the film surface is low compared to that of the rear side of the film opposite to the outer surface, which makes the temperature distribution uneven with respect to the direction of the thickness of the film. This also impedes the growth of a large single crystal.
To cope with this problem, it is proposed to irradiate and scan the thin film with a laser beam in such a way that the film surface is coated with a cooling medium such as polyethylene glycol which enhances the heat radiation from the film surface.
However, even if the thin film is coated with the cooling medium, the temperature distribution of the thin film is different from that of the beam spot of the laser which scans the thin film to melt and recrystallize the film. More precisely, the laser beam spot has a Gaussian temperature distribution. When this laser beam scans the semiconductor thin film surface along a certain direction, the profile of the temperature distribution represented by the contour lines becomes longitudinal so that the tail portion thereof is elongated toward the direction opposite to the scanning direction. As a result, the temperature difference between the beam trace of the spot center and that of the periphery of the spot becomes large, which causes generation of cracks at the time of recrystallization.